Particles don't "bend" with displacement
Several things might come into account. The main two I can think of are:

Wrong modifier order

If you pout your displace modifier after the particles modifier, then it means that the particles are added when the mesh is flat, and only then the mesh is displaced. The right thing to do is first displace, then put the particles on it. Modifiers order matters.



Use Modifier Stack

If you use a subdivision modifier, you might check that in the particle's emission pannel.